Eddy current flaw detection system

ABSTRACT

An automatic eddy current flaw detection system including a measuring instrument with a chart recorder and a flaw sensing device having an eddy current probe in a housing attached to the work piece and driven by a reversible motor along a helical path into an opening in the work piece. The housing contains a standard calibration unit which the probe passes and thereby allows for instrument standarization facilitating the reproducibility of inspection conditions.

United States Patent 91 Rogel et al.

[ EDDY CURRENT FLAW DETECTION SYSTEM Inventors: Albert P. Rogel, 2655Ellenbrook Drive, Rancho Cordova, Calif. 95670; Joseph J. Scalese, 9507Bullion Way, Orangevale, Calif. 95662 H' Ti IODY CUIIIN'! 1 Feb. 27,1973 2,684,464 7/1954 Hastings et al. ..324/37 3,209,243 9/1965 Walterset al. ..324/37 3,327,205 6/1967 Wood et al. ..324/37 3,419,797 l2/l968Libby ..324/40 Primary ExaminerRobert J. Corcoran AttorneyHarry A.Herbert, Jr. and Henry S. Miller, Jr.

[57] ABSTRACT An automatic eddy current flaw detection system includinga measuring instrument with a chart recorder and a flaw sensing devicehaving an eddy current probe in a housing attached to the work piece anddriven by a reversible motor along a helical path into an opening in thework piece. The housing contains a standard calibration unit which theprobe passes and thereby allows for instrument standarizationfacilitating the reproducibility of inspection conditions.

9 Claims, 4 Drawing Figures PATENTEB FEB 2 7 I975 SHEET 10F 4 1 EDDYCURRENT FLAW DETECTION SYSTEM BACKGROUND OF THE INVENTION This inventionrelates generally to a method and system of flow detection in metals andmore particularly to an eddy current automatic scanner for locatingfatigue cracks and other discontinuities in metal structures.

Eddy current probes have been used and accepted as a means for locatingflows in metal structures during non destructive testing. With currentlyavailable devices the probe is hand held and inserted into a hole in themetal. A small wire wound coil attached at the end of the probe isenergized with a high frequency alternating current which induces eddycurrents in the wall of the hole. Any cracks or discontinuities in thehole wall effect the induced eddy current field which reacts with thebalanced field of the core and translates the effect into meter readingson an eddy current power supply and measuring instrument.

This method of flaw detection, while extremely efficient from atheoretical point of view is, from a practical standpoint inefficient aswell as unreliable and cumbersome. The hand held eddy current probecannot provide valid reproducible data for recording which is essentialwhere the holes examined are tested as part of a periodic inspection. Anexample of this would be certain critical mounting holes on aircraftwhich are required to be tested according to a predetermined schedule.The hand held probe is incapable of producing any test results thatcould be preserved and compared from one inspection to the next, wheresuch results would show relative wear in the hole.

In addition to the aforementioned disadvantages of the prior art, thehand held method is extremely tedious from the inspectors standpoint.During a hole test the inspector must continually evaluate test signalsfrom many variables including irregular rotation of the probe, tiltingof the probe, out-of-the-round holes, rough holes and constant meterdeflections, in order to establish the validity of the meter readings.

It appears obvious that the hand held eddy probe is of only marginalvalue even when used by a highly skilled technician and the pace andduration of tests thus performed is limited to thepatience and enduranceof the operator.

SUMMARY OF THE lNVENTlON The invention consists of an automatic eddycurrent inspection system that can be utilized to detect fatigue cracksor discontinuities in holes and openings in aluminum, steel or othermetal structures. The invention provides a universal mounting for aneddy current probe. The probe is driven through a gear arrangement by areversible electric motor attached to the probe housing. The probehousing is attached to the universal mounting and is internally threadedto mate with an external thread on a spindle containing the probe. Asthe motor drives the spindle it moves into and out of the housing due tothe arrangement of the threads on the spindle and the housing.

The end of the housing nearest the universal mounting contains ananertured calibration disc. As the probe is driven through the disc arecording is made of its reading through the disc. Changes inelectronics of the system can be made to duplicate a preexisting testwhere the calibration data is available.

The calibration disc is constructed having a notch located along itsinner circumference. The notch simulates a crack ordiscontinuity andtests the electronics to insure that they are functioning properly. Thenotch also is used in conjunction with a marking indicator representedby a micro switch located outside the housing and riding on the spindleextension. A groove in the spindle extension causes the switch to closeonce each time the spindle turns 360. The marking switch is connected tothe chart recorder of the power supply measuring unit and causes a markto be placed along the edge of the chart paper after each revolution ofthe spindle.

The combination of the notched calibration disc and the markingindicator allows the inspector to determine immediately not only that aflaw has been detected but the exact location of the flaw as well.

It is therefore an object of this invention to provide a new andimproved eddy current system for the non destructive testing of metals.

It is another object of this invention to provide a new and improvedsystem for flaw detection.

It is a further object of this invention to provide an eddy current flawdetection system that is more accurate than any hitherto known.

It is still another object of the invention to provide an eddy currentflaw detection system that allows repeatable inspections and thecomparison of inspection data.

It is still a further object of the invention to provide a flaw detectorthat will record the size, length and orientation of a flaw.

It is another object of the invention to provide a eddy current flawdetector having a calibration standard.

It is another object of the invention to provide an eddy current flawdetector that is completely automatic.

It is another object of the invention to provide an automatic eddycurrent flaw detector that will detect flaws faster and with lessoperator strain than any presently known.

It is another object of the invention to provide an eddy current flawdetector that will inspect multiple layers of metal without resetting orrecalibration.

It is another object of the invention to provide an automatic eddycurrent flaw detector system that supplies a permanent record to aid indefect interpretation.

It is another object of the invention to provide an automatic eddycurrent flaw detector that requires a less skilled inspector than thosesystems presently available.

It is another object of the invention to provide an automatic eddycurrent flaw detector which is economi-- cal to produce and utilizesconventional, currently available components that lend themselves tostandard mass production manufacturing techniques.

These and other advantages, features and objects of the invention willbecome more apparent from the following description taken in connectionwith the illustrative embodiment in the accompanying drawings.

DESCRIPTION OF DRAWINGS FIG. 1 is a schematicdrawing of the system ofthe invention.

FIG. 2 is a pictorial representation showing a simulated fatigue crackand an inspection record thereof.

FIG. 3 is a side elevational view of the invention partly in section.

FIG. 4 is an exploded view of the invention.

DESCRIPTION OF PREFERRED EMBODIMENT Referring now to FIG. 1 where theeddy current flaw detection system is disclosed in its entirety. Theeddy current probe and its drive mechanism is shown generally at 10. Thepower supply measuring instrument is shown generally at 12 and therecorder shown generally at 14.

In operation, the probe and drive mechanism is attached to a work piece16 by bolt 18 having an adaptive head 20 and a nut 22. The nut pressesagainst a washer 24 and holds the universal mounting bracket 26 inplace. Leveling screws 27 level the bracket 26 to provide accuratealignment between the opening 29 and the probe 28.

The eddy current probe 28 is attached to the power supply-measuringinstrument by line 30 while the drive motor 32 is attached via line 34.The drive motor is controlled from the three position switch 36 thatprovides OFF, Rotate Clockwise and Rotate Counterclockwise positions.

The power supply-measuring instrument 12 has a DC Microampere meterwhich measures the current flow in the probe caused by an imbalance in abridge circuit located in the probe. The bridge becomes unbalanced whenthe matched magnetic fields are mismatched by a flaw in the metal. Theinstrument 12 has means provided for calibrating a standard as will beexplained hereinafter.

Connected to the instrument 12, by line 38, is a high speed chartrecorder 14. The stylus 40 records the current flaw shown by the needle42 of the instrument 12. In addition, a marking switch 44 mounted at thebase of the probe and drive mechanism 10 sends a signal via line 46 tothe recorder 14 and causes a mark 48 to be recorded each time the probeis rotated 360.

FIG. 2 shows a comparison between a hole in an aircraft wing attachmentfitting and the recorded inspection data. The scan of the hole was takenfrom the left to the right as one looks at the drawing. The probe passesthe standard calibration disc 50 which has a notch 52 and leaves thescan pattern 54, as the probe follows a helical path and moves ahead in0.025 inch steps. The recording 56 shows no unusual purtubations as itpasses through the wing skin 58 until it reaches the wing attachmentfitting 60. As the probe approaches the fatigue crack 62 in the wingattachment fitting 60 the bridge in the electrical system becomesunbalanced and the probe tends to draw more current when rotated intothe vicinity of the crack as shown by the large vertical deviations ofthe recorded data. As the crack closes and becomes shallower, thedeviations from normal of the data decline. This decline is generally inproportion to the depth of the crack.

An important feature of the invention is the marking system shown on thedata tape 56. The markings 64 indicate that the probe has turned 360.With these marks it is then easily determined where in the hole thecrack is located since the standard disc is oriented and has a notchtherein to provide orientation data.

FIG. 3 shows the probe, drive and housing. The universal adapter plate66 is held against a work piece 68 by the nut-bolt arrangement 70. Acable 72 supplies electrical current to the reversible synchronous motor74. The motor 74 is mounted on the gear housing 76 and drives the spurgears 78 and 80. Spur gear 80 is affixed to the spindle 82 and heldsecure by the spindle drive key 84. the spindle is mounted in thespindle housing 86 through a threaded portion 88 which engages a similarportion in the housing 86.

The expandable eddy probe 90, similar to that made by the IdealSpecialty Company of Tulsa Okla. Model No. 6800, has a split head 91 andis expandable to adjust to various size holes. The probe is secured inthe spindle by the probe holder 92. The probe holder (92) is retained bytwo rubber O-rings 94, 96 that allow the probe to slide longitudinallyin the spindle cavity should the probe strike an obstruction during theinspection.

Attached to the probe is a signal cable 98 which is held in position atthe base of the spindle by the cable holder 100.

To secure the spindle housing 86 to the universal mounting plate 66having levelling screws 67, an attaching adapter 102 is provided securedto the mounting plate and adapted to threadingly engage the spindlehousing. A calibrated standard disc 104 is mounted in a recess in theattaching adapter 102 to allow for calibrating the probe before itenters the workpiece 68.

An exploded view of the probe holder and scanning mechanism is shown inFIG. 4. The reversible synchronous motor 104 is held to the gear housing106 by four bolts one of which is shown at 108. The gear housingcontains spur gears 110 and 112. Gear 110 mates with the flat sidedshaft 114 of the motor 104. Gear 112 slides on the spindle 116 and isheld in place by the spindle drive key 118 which slides in the slot onthe gear and the slot 120 on the spindle 116. The gear housing isprovided with a cover 122 having a gasket 124 therebetween. The cover isheld in place by six screws exemplified by 126. Marker switch 128 isattached to-the gear housing cover and the switch arm 129 rides on thespindle (116) extension. Each time the switch arm drops into the groove120, the switch closes leaving a mark on the data record. The signalcable 130 is held in the end of the spindle by the holder 132.

The gear housing 106 is attached to the spindle housing 136 by fourscrews exemplified by 134. The spindle housing is held by the adaptersection of the universal mounting 138.

Within the spindle housing 136 is the spindle 116 and within the spindleis the probe 144. The probe 144 is secured in the spindle 116 by theprobe holder 146. The holder (146) consists of two half pieces 148, 150which clamp around the probe and are held together by two O-rings 152,154 which have a tight sliding fit against the inner walls of spindle.Centering pin is provided to accurately align spindle housing 136 withthe centerline of the hole being inspected.

Having thus described our system for eddy current flaw detection weclaim the following as our invention:

1. An apparatus to perform the non-destructive testing of metalscomprising: a universal planular mounting bracket; a housing; attachingmeans mounted on the universal mounting bracket for removably securingthe housing to said bracket; spindle means mounted in said housing andadapted to have a sliding engagement with the housing along itslongitudinal axis; an eddy current test instrument secured within saidspindle means and extending along the longitudinal axis thereof; gearhousing means secured to said housing; drive means secured to the gearhousing, gears secured to the said drive means and said spindle throughwhich the drive means may cause the spindle to achieve a rotary motion.

2. An apparatus to perform the non-destructive testing of metalsaccording to claim 1 including: centering means adapted to be removablysecured in the attaching means for positioning the universal bracketmeans on a work piece.

3. An apparatus to perform the non-destructive testing of metalsaccording to claim 1 wherein: said housing is provided with threads onan internal surface and said spindle means is provided with threads onan external surface wherein said external threads engage said internalthreads causing the spindle to move with a helical motion upon beingrotated.

4. An apparatus to perform the non-destructive testing of metalsaccording to claim 1 including, means for securing the test instrumentcomprising: a pair of blocks which when placed together along theirlongitudinal axis form a cylinder, and having an opening therein tograsp a test instrument; and a pair of O-rings located around saidblocks to hold the blocks in position and to engage the side walls ofsaid spindle in a tight sliding fit whereby said testing instrument willslide along the walls of the spindle if a force is applied thereto.

S. An apparatus to perform the non-destructive testing of metalsaccording to claim 1 wherein, one end of said test instrument is splitto allow for adjustment of the instrument to test pieces of varyingsize.

6. An apparatus to perform the non-destructive testing of metalsaccording to claim 1 including, a calibration standard mounted in theattaching means and through which the test instrument must pass prior totesting.

7. An apparatus to perform the non-destructive testing of metalsaccording to claim 6 wherein, said calibration standard is provided withmeans for perturbing the test instrument output.

8. An apparatus to perform the non-destructive testing of metalsaccording to claim 1 wherein, said drive means is a reversiblesynchronous motor.

9. An apparatus to perform the non-destructive testing of metalsaccording to claim 1 including, a normally open switch mounted on saidgear housing said switch having an arm in contact with the spindlemeans, said spindle means having a switch activating means thereonwhereby each time the spindle means completes one revolution said switchwill momentarily close.

1. An apparatus to perform the non-destructive testing of metalscomprising: a universal planular mounting bracket; a housing; attachingmeans mounted on the universal mounting bracket for removably securingthe housing to said bracket; spindle means mounted in said housing andadapted to have a sliding engagement with the housing along itslongitudinal axis; an eddy current test instrument secured within saidspindle means and extending along the longitudinal axis thereof; gearhousing means secured to said housing; drive means secured to the gearhousing, gears secured to the said drive means and said spindle throughwhich the drive means may cause the spindle to achieve a rotary motion.2. An apparatus to perform the non-destructive testing of metalsaccording to claim 1 including: centering means adapted to be removablysecured in the attaching means for positioning the universal bracketmeans on a work piece.
 3. An apparatus to perform the non-destructivetesting of metals according to claim 1 wherein: said housing is providedwith threads on an internal surface and said spindle means is providedwith threads on an external surface wherein said external threads engagesaid internal threads causing the spindle to move with a helical motionupon being rotated.
 4. An apparatus to perform the non-destructivetesting of metals according to claim 1 including, means for securing thetest instrument comprising: a pair of blocks which when placed togetheralong their longitudinal axis form a cylinder, and having an openingtherein to grasp a test instrument; and a pair of O-rings located aroundsaid blocks to hold the blocks in position and to engage the side wallsof said spindle in a tight sliding fit whereby said testing instrumentwill slide along the walls of the spindle if a force is applied thereto.5. An apparatus to perform the non-destructive testing of metalsaccording to claim 1 wherein, one end of said test instrument is splitto allow for adjustment of the instrument to test pieces of varyingsize.
 6. An apparatus to perform the non-destructive Testing of metalsaccording to claim 1 including, a calibration standard mounted in theattaching means and through which the test instrument must pass prior totesting.
 7. An apparatus to perform the non-destructive testing ofmetals according to claim 6 wherein, said calibration standard isprovided with means for perturbing the test instrument output.
 8. Anapparatus to perform the non-destructive testing of metals according toclaim 1 wherein, said drive means is a reversible synchronous motor. 9.An apparatus to perform the non-destructive testing of metals accordingto claim 1 including, a normally open switch mounted on said gearhousing said switch having an arm in contact with the spindle means,said spindle means having a switch activating means thereon whereby eachtime the spindle means completes one revolution said switch willmomentarily close.